Curved cuts within a transparent material are generated, in particular, in laser-surgical methods, especially in ophthalmic surgery. This involves focusing treatment laser radiation within the tissue, i.e. beneath the tissue surface, so as to form optical breakthroughs in the tissue.
In the tissue, several processes initiated by the laser radiation occur in a time sequence. If the power density of the radiation exceeds a threshold value, an optical breakthrough will result, generating a plasma bubble in the material. After the optical breakthrough has been generated, said plasma bubble grows due to expanding gases. If the optical breakthrough is not maintained, the gas generated in the plasma bubble will be absorbed by the surrounding material, and the bubble disappears again. However, this process takes very much longer than the forming of the bubble itself. If a plasma is generated at a material boundary, which may also be located within a material structure as well, material will be removed from said boundary. This is then ref erred to as photo ablation. In connection with a plasma bubble which separates material layers that were previously connected, one usually speaks of photo disruption. For the sake of simplicity, all such processes are summarized here by the term optical breakthrough, i.e. said term includes not only the actual optical breakthrough, but also the effects resulting therefrom in the material.
For a high accuracy of a laser surgery method, it is desirable to guarantee high localization of the effect of the laser beams and to avoid collateral damage to adjacent tissue as far as possible. It is therefore common in the prior art to apply the laser radiation in pulsed form, so that the threshold value for the power density of the laser radiation required to cause an optical breakthrough is exceeded only during the individual pulses. In this regard, U.S. Pat. No. 5,984,916 clearly shows that the spatial extent of the optical breakthrough (in this case, of the generated interaction) strongly depends on the pulse duration. Therefore, high focusing of the laser beam in combination with very short pulses allows to place the optical breakthrough in a material with great point accuracy.
The use of pulsed laser radiation has recently become established practice particularly for laser-surgical correction of visual defects in opthalmology. Visual defects of the eye often result from the fact that the refractive properties of the cornea and of the lens do not cause optimal focusing on the retina.
U.S. Pat. No. 5,984,916 mentioned above, as well as U.S. Pat. No. 6,110,166, describe methods of producing cuts by means of suitable generation of optical breakthroughs, so that, ultimately, the refractive properties of the cornea are selectively influenced. A multitude of optical breakthroughs are sequentially arranged such that a lens-shaped partial volume is isolated within the cornea of the eye. The lens-shaped partial volume which is separated from the remaining corneal tissue is then removed from the cornea through a laterally opening cut. The shape of the partial volume is selected such that, after removal, the shape and, thus, the refractive properties of the cornea are modified such that the desired correction of the visual defect is effected. The cuts required here are curved, which makes a three-dimensional shifting of the focus necessary. Therefore, a two-dimensional deflection of the laser radiation is combined with simultaneous shifting of the focus in a third spatial direction.
When producing a cut by a series of optical breakthroughs in the material, an optical breakthrough is generated several times faster than it takes until a plasma generated therefrom is absorbed by the tissue again. It is known from the publication by A. Heisterkamp, et al., Der Opthalmologe, 2001, 98:623-628, that after an optical breakthrough has been generated in the cornea of the eye a plasma bubble grows at the focal point where the optical breakthrough was generated, which plasma bubble reaches a maximum size after a few us and then almost completely collapses again. This then leaves only small residual bubbles. Said publication states that joining of growing plasma bubbles reduces the quality of the cut. Therefore, it suggests a method of the above-mentioned type, wherein individual plasma bubbles are not generated directly next to each other. Instead, a gap is left between sequentially generated optical breakthroughs, which breakthroughs are generated along a spiral-shaped path. The gap is filled, in a second pass, through the spiral with optical breakthroughs and with plasma bubbles resulting therefrom. This is intended to prevent joining of adjacent plasma bubbles and to promote the quality of the cut. In the spiral described by Heisterkamp et al., the distance of the generated optical breakthroughs inevitably increases with the spiral windings.
As an alternative to the approach described in the cited publication, it could also be contemplated to make the time interval between subsequently generated optical breakthroughs so large that the plasma bubble of one optical breakthrough has already collapsed before the next optical breakthrough is generated. However, this would considerably slow down the production of the cut.
Generating cuts quickly is desirable not only for convenience or in order to save time; bearing in mind that movements of the eye inevitably occur during ophthalmic operations, quick generation of cuts also improves the optical quality of the result thus achieved and reduces the requirement to track eye movements.
Therefore, it is an object of the invention to improve a method and a device of the above-mentioned type such that generating good-quality cuts requires as little time as possible.